Low protein grain results in the loss of income to the grower through price discounts, but can be avoided by late-season nitrogen fertilizer applications.
Low protein in durum is primarily related to low nitrogen (N) availability, but is often associated with other factors such as yield, irrigation, soil texture, previous crop, variety, and weather.
The effect of these other factors on grain protein may be indirect since most of them also influence N availability. Factors which can influence grain protein content are explained below.
Seasonal N fertilizer rate can affect grain protein content and yield. However, the peak grain protein will be attained at a higher nitrogen rate than the peak grain yield.
Late season N fertilizer is more effective than early season applications in increasing grain protein content.
The optimal timing of late season nitrogen to increase grain protein content is between flowering and two weeks thereafter. An application of 38 pounds of N per acre will increase grain protein content by about 1 percent.
N fertilization efficiency can affect grain protein content. N applied in the irrigation water is only applied as efficiently as the water is applied.
Anhydrous ammonia injected in high pH irrigation water is particularly subject to loss. Broadcast urea and ammonium fertilizers are subject to volatilization losses before incorporation with irrigation water.
High grain yields can result in decreased grain protein due to protein dilution. The amount of protein on a per acre basis could be the same for high yields of low protein grain as low yields of high protein grain. In the case of high yields, a given amount of protein may be diluted in a higher amount of starch in the kernel, resulting in a lower protein concentration.
Frequent irrigation impact
Frequent irrigation can decrease grain protein due to increased yield or reduced N availability. N availability may be reduced due to waterlogged conditions, whereby lack of oxygen inhibits N uptake, or due to losses of N through leaching or denitrification (gaseous loss).
Coarse textured soils can produce low protein grain since they are likely to have high leaching potential and low N content. Soil N usually accounts for about half of the crop N uptake and fertilizer accounts for the other half.
The previously grown crop can influence durum protein content due to the effect on soil N. Alfalfa increases soil Nitrogen available to the next crop. Many vegetable crops are heavily fertilized and result in high levels of residual Nitrogen. Some crops including small grains, corn, and sorghum leave a residue that depletes soil N during decomposition of the residue.
Certain durum varieties are genetically predisposed to low protein content. However, most durum varieties grown in Arizona are capable of attaining 13 percent protein if N availability is sufficiently high.
The growing season weather appears to influence grain protein content in some years due to the effect on yield. Low protein content seems to be more common in high-yielding years.
Low protein content in durum can be avoided in most cases by being aware of the factors that influence protein content. An application of 30-60 pounds of N per acre (or more) between flowering and two weeks thereafter may be necessary to achieve 13 percent protein.
The lower stem nitrate test is a good indicator of the N status of the crop and the need for late season N fertilizer.